Conventional fire suppression systems delivering dry chemical powder carried by compressed inert gas lack the versatility of standard liquid fire suppression systems such as water sprinkler systems. This is because dry chemical powder carried in a gas stream will separate from the transporting gas if the velocity falls below that value required to carry it along, due to the effect of gravity on the powder particles. It will also separate when the direction of travel is changed, as in passing through an elbow, due to the inertia of the powder particles, resulting in a higher concentration of powder on the far side of the pipe immediately downstream from the elbow. These unequal distributions of powder in such systems is fully documented in Guise U.S. Pat. No. 2,708,605.
Dry chemical powder carried in a gas stream must be delivered in effective quantities to each different fire location. Underwriters' Laboratories have verified tests demonstrating the desirability of imposing the requirement that the entire charge of dry powder must be delivered within a few seconds, for example, in specified minimum quantities to different cooking appliances in a restaurant kitchen. Thus, in a typical system, the exhaust duct nozzle must deliver 3.9 pounds, and plenum nozzles in the exhaust hood plenum must deliver 5.1 pounds. A nozzle protecting a charbroiler must deliver 4.0 pounds, and nozzles protecting other appliances such as deep fat fryers, small griddles and broiler grills must deliver 1.9 pounds, all within the minimum required discharge time.
In order to assure the delivery of these minimum quantities of dry chemical powder through the conventional piping system the powder-gas stream is divided in substantially equal portions and then subdivided again and again at equal-branching Tees. These forked or multiple-bifurcated piping systems have been considered necessary to assure substantially even division of the gas powder stream. In order to produce an unbalanced division and deliver more powder through one branch of the Tee, for example, to protect one or two appliances that require more powder, the opposite branch must have its flow hindered by the use of longer lengths and/or smaller sizes of pipe and added elbows. These are the present means of controlling the distribution of powder to assure that each appliance, deep fat fryer, grill, griddle, and broiler, receives at least its required minimum volume of fire suppressing powder while the exhaust hood, plenum and ducts were also assured of receiving their required minimum volumes of fire suppressing powder.
Piping is further complicated by the effect of inertia, as mentioned above, requiring that a branching Tee immediately downstream from an elbow must be oriented with its horns at right angles to the pipe upstream before the elbow or that a minimum distance equal to approximately 20 pipe diameters be placed between the elbow and the Tee, to allow the powder in the stream to approach homogeneity.
The result of these multiple-bifurcated branching piping systems with the extra pipe length and sizes, and added elbows has been increased cost and reduced efficiency. The multiple branches and added elbows actually duplicate runs of piping in horizontal directions as the powder-gas stream is delivered to a central point and then branched to first bifurcations, travelling thence to second and third bifurcations before it reaches the delivery nozzles. Moreover, the insertion of so many extra forking, branching Tees, elbows and otherwise unnecessary length of piping absorbs kinetic energy from the advancing gas-powder stream, building powder deposits in dead spots inside the piping system by reducing its velocity below that value which is required to suspend and carry the powder particles along, resulting in reduction of powder delivered to the nozzles at the end of the line. This often wastes the fire suppressing powder, because excess quantities must be delivered to various sites in order to assure that minimum quantities reach particular sites, such as more remote hazards, due to the inherent lack of better control of powder distribution.